Disrupted sleep could accelerate cancer growth due to its negative effects on the immune system, according to new research in mice.

In the study, published in the journal Cancer Research, researchers split mice into two groups. One group of mice was allowed to sleep peacefully during the day (mice are nocturnal and typically sleep in the daytime), while the other mice had their sleep disturbed every two minutes with a motorized brush that swept through their cages; these mice were forced to wake up and go back to sleep.

The mice experienced these settings for seven days before being injected with cells from one of two different kinds of tumors. After being injected, all the mice went on to develop tumors within nine to 12 days; the researchers examined the tumors four weeks later.

The tumors that came from the mice whose sleep was disturbed were twice as big as the ones from the mice who were not disturbed, researchers found.

And in another similar experiment with disrupted sleep in mice, but involving implanting tumor cells in the mice's thigh muscles, researchers found that the mice with disrupted sleep had more aggressive tumors that spread beyond the tissue and into the muscle and bone.

The culprit seems to be sleep's effects on the immune system. Researchers found that the well-rested mice had more immune system cells called M1-type tumor-associated macrophages (TAMs), which are known to promote the immune system and stop cancer cells. However, the sleep-disrupted mice had more immune cells called M2-TAMs, which are known to actually hinder the immune system and promote tumors through blood vessel growth.

"This study offers biological plausibility to the epidemiological associations between perturbed sleep and cancer outcomes," study researcher Dr. David Gozal, M.D., chairman of pediatrics at the University of Chicago Comer Children's Hospital, said in a statement. "The take home message is to take care of your sleep quality and quantity like you take care of your bank account."